The present invention relates to a bearing arrangement for impellers of a compressor and a turbine of an exhaust gas turbocharger in a compressor housing having a housing bore.
A bearing arrangement of this general type is known from European patent No. 0,121,670. In this known bearing arrangement, the support of the rotor shaft takes place via two ball bearings that are inserted in a housing bore such that, inside the hub that is arranged centrally in the inlet housing of the compressor, the outer races rest directly against the bore wall.
Because of the relatively rigid bearing of the rotor, in the known arrangement, the danger exists that the critical rotational speed of the rotor shaft occurs in the proximity range of the maximum rotational speed of the turbocharger which requires a precise balancing of the impellers with the rotor shaft. Despite careful balancing, higher bearing forces, in the known arrangement, may occur at the ball bearings that can result in premature fatigue or the destruction of the ball bearings.
Although it is possible, by shortening the overall length of the rotor, to shift the critical rotational speed out of the range of the maximum rotational speed, this requires that the impellers be arranged as close together as possible. This results in the overall efficiency of the turbocharger being impaired because of the increased heat transfer from the turbine side to the compressor side.
The known bearing arrangement also has the disadvantage that increases of the rotational speed for achieving a higher compressor pressure ratio result in a decrease of operational reliability. Alternative to increasing the rotational speed, an enlargement of the impeller diameters, while keeping the maximum rotational speed the same, results in an increase of the moment of inertia of the rotor and as a result in a lasting deterioration of the response behavior.
The present invention therefore has an objective of improving the known bearing arrangement in such a way that, with low requirements with respect to the quality of the balancing, a rising of the compressor pressure ratio is achieved with small impellers with low inertia, without increasing the bearing load.
This and other objectives are achieved in the present invention by providing in a bearing arrangement for impellers of a compressor and a turbine of an exhaust gas turbocharger with at least two ball bearings separated a distance from one another, with at least one of the ball bearings being supported in the housing bore and enclosing at least one of a gap-shaped damping space acting as a hydraulic cushioning means and a radially active spring body.
By means of the invention, an elastic and damping support of the rotor shaft and of the impellers mounted on it is achieved in the compressor housing, whereby the critical rotational speed is shifted out of the range of the maximum rotational speed of the turbocharger. In this case, the means that are provided for the damping and the elastic support are influenceable in such a way that, after a raising of the rotational speed limit of the turbocharger, the critical rotational speed will still be outside the range of the new maximum rotational speed of the turbocharger. This has the advantage that an increase of the compressor pressure ratio can be achieved by means of small impellers with low inertia and without any significant increase of the bearing load.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.